1. Field of the Invention
The present invention relates to active water compatible energy curable compositions containing a maleimide derivative, useful for preparing various coatings, printing inks, surface finishes, moldings, laminated plates, adhesives, and binders. More specifically, the present invention relates to active water compatible energy curable compositions, which can be cured in the absence of a photoinitiator with an irradiation source of practical intensity and energy value.
2. Description of Related Art
An active energy curable composition polymerized under irradiation of active energy such as thermal energy, ultraviolet light, visible light, and the like, has an advantage of being rapidly cured. Active energy curable compositions are widely used as paints, inks, adhesives, coatings, and the like. However, conventional ultraviolet active energy curable compositions cannot initiate polymerization alone upon irradiation with an energy source; it is therefore necessary to use a photoinitiator. When photoinitiators are used in large quantities, curing progresses rapidly, which encourages the use of large quantities of photoinitiator.
Photoinitator compounds having an aromatic ring are used in general because they effectively absorb ultraviolet light. However, these compounds cause problems such as the yellowing of the cured materials upon addition of heat or light. Moreover, low molecular weight energy curable monomers and oligomers, commonly used as photoinitiators because of their solubility a property necessary to initiate photopolymerization effectively, unfortunately have high vapor pressures. Therefore, they tend to give off unpleasant odors at temperatures ranging from room temperature to 150° C. Because infrared light, for example, is generated from an ultraviolet energy source, active energy curable compositions are heated substantially upon contact with such light sources. The heating problem is magnified when the ultraviolet light lamps are arranged and used in a side-by-side fashion. The unpleasant odors given off from the photoinitiator result in an unhealthy working environment.
Unreacted or decomposed photoinitiators remain behind in conventional energy curable compositions even after exposure to irradiation by the active energy cure source. These unreacted or decomposed photoinitiators cause problems such as changing the color of the cured film to yellow, unpleasant odors, and the like, when the cured film is exposed to heat or light. For example, when a material at high temperature, such as a thermal head, contacts an active energy curable composition comprising photoinitiator, strong unpleasant odors are given off. Finally, when these cured compositions are contacted by water after irradiation; unreacted photoinitiator is exuded; therefore causing the active energy curable composition to be unsuitable for food packaging applications.
In solving some of these problems, the prior art presents many options. For instance, JP-A-58-89609 discloses an energy curable resin comprising a polymer with polymerizable unsaturated acrylic group and an organic solvent-soluble styrene containing an acrylic thermoplastic resin that does not need a photoinitiator.
WO 89/05827 teaches photopolymerizable adhesive compositions comprising a copolymer of methacrylate monomer and/or methyl acrylate and a photopolymerizable monomer. These photocurable compositions, however, cannot be sufficiently cross-linked by practical irradiation energy sources.
U.S. Pat. No. 5,446,073 and Polymer Preprints, Vol. 37, N 0.2, pp. 348-49, 1996 disclose a photopolymerizing method in which maleimide type materials are mixed with vinyl ethers and acrylates to produce a tough film. The polymerization mechanism involves a charge-transfer complex, which is formed by an electron acceptor and an electron donor. However, many of the maleimides are solid and are hardly dissolved in acrylates.
Polymer Letters, Vol. 6, pp. 883-88,1968 reports that maleimide derivatives can be polymerized in the absence of photoinitiators under irradiation by ultraviolet light. Japanese Patent Applications JP-A-61-250064, JP-A-62-64813, and JP-A-62-79243 teach energy curable compositions comprising maleimide derivatives such as alkylmaleimides and arylmaleimides. However, these maleimide derivatives show low photoinitiator properties, therefore making it necessary to use substantial amounts of photoinitiator in the maleimide compositions.
U.S. Pat. No. 3,920,618 and Japanese Patent applications JP-A-50-123138 and JP-A51-47940 disclose photopolymerizable polymers having an α-aryl substituted maleimide group at a side chain. It is well known that these pendant type maleimides can be crosslinkable by ultraviolet irradiation (i.e. 2+2 photocycloaddition reaction). U.S. Pat. No. 4,079,041 and Europe Patent 21019 teach polymers having side chain type maleimide groups with alkyl substituents. However, these pendant type maleimides cannot be used to form linear polymers by photopolymerization. Therefore they are most commonly used to prepare negative printing plates. In addition, the photocross-linking dimerization reaction takes a rather long time (several tens seconds to several minutes) even with an excess amount of irradiation energy.
Polymer Materials Science and Engineering, Vol. 72, pp. 470-72, 1995 and Proceedings of RadT ch Europe 95, pp. 34-56, 2995 disclose photocurable compositions comprising maleimide derivatives as electron acceptors and vinyl ethers as electron donors. The photopolymerizable compositions 1,4-bis(vinyloxymethyl)cyclohexane and N-cyclohexylamalemide or 4-hydroxybutyl vinyl ether and N-(hydroxyalkyl) maleimide, illustrated in these documents are polymerized upon ultra violet irradiation in the absence of a photoinitiator. However, hardening of the coated films does not occur; i.e. the coated films maintain liquid states after ultraviolet irradiation.
WO 98/07759 describes energy curable compositions wherein water-soluble maleimides are copolymerized with acrylates in the absence of water to produce a cured film.
The polymerizing methods described above share numerous problems, which can be summarized as the need for high irradiation intensity to cure sufficiently; the maleimide derivatives being solid at ambient temperature which does not suggest whether they are or can be homo-polymerized upon irradiation in the absence of a photoinitiator; difficulty in obtaining cured coating with practical properties and given the wide range of curable composition disclosed; the need for higher irradiation energy than practical for cross-linking (photodimerization). However, none of these references describe active energy curable compositions containing water or energy curable compositions that are water compatible.
It is an object of the present invention to provide active water compatible energy curable compositions which do not contain photoinitiator, cause unpleasant odors upon curing or cause yellowing, or exude materials from the cured film upon contact with water or solvent. Another object of the present invention is to provide an active water compatible energy curable composition which be photopolymerized by an energy source of practical intensity and energy value and results in coatings that exhibit cure rates, gloss, hardness and solvent resistance values comparable to those of conventional energy cure systems employing photoinitiators.